C++ Primer Plus (P42) pptx

So far the class examples have used the keywords public and private to control access to class members. There is one more access category, denoted with the keyword protected. The protected keyword is like private in that the outside world can access class members in a protected section only by using public class members. The difference between private and protected comes into play only within classes derived from the base class. Members of a derived class can access protected members of a base class directly, but they cannot directly access private members of the base class. So members in the protected category behave like private members as far as the outside world is concerned but behave like public members as far as derived classes are concerned. For example, suppose the Brass class declared the balance member as protected: class Brass { protected: double balance; }; Then the BrassPlus class could access balance directly without using Brass methods. For example, the core of BrassPlus::Withdraw() could be written this way: void BrassPlus::Withdraw(double amt) { if (amt < 0) cout << "Negative deposit not allowed; " << "withdrawal canceled.\n"; else if (amt <= balance) // access balance directly balance -= amt; else if ( amt <= balance + maxLoan - owesBank) { double advance = amt - balance; owesBank += advance * (1.0 + rate); cout << "Bank advance: $" << advance << endl; cout << "Finance charge: $" << advance * rate << endl; Deposit(advance); This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. balance -= amt; } else cout << "Credit limit exceeded. Transaction cancelled.\ n"; } Using protected data members may simplify writing the code, but it has a design defect. For example, continuing with the BrassPlus example, if balance were protected, you could write code like this: void BrassPlus::Reset(double amt) { balance = amt; } The Brass class was designed so that the Deposit() and Withdraw() interface provided the only means for altering balance. But the Reset() method essentially makes balance a public variable as far as BrassPlus objects are concerned, ignoring, for example, the safeguards found in Withdraw(). Caution Prefer private to protected access control for class data members, and use base-class methods to provide derived classes access to base-class data. However, protected access control can be quite useful for member functions, giving derived classes access to internal functions that are not available publicly. Abstract Base Classes So far you've seen simple inheritance and the more intricate polymorphic inheritance. The next step in increasing sophistication is the abstract base class, or ABC. Let's look at some programming situations that provide the background for the ABC. Sometimes the application of the is-a rule is not as simple as it might appear. This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. Suppose, for example, you are developing a graphics program that is supposed to represent, among other things, circles and ellipses. A circle is a special case of an ellipse; it's an ellipse whose long axis is the same as its short axis. Therefore, all circles are ellipses, and it is tempting to derive a Circle class from an Ellipse class. But once you get to the details, you may find problems. To see this, first consider what you might include as part of an Ellipse class. Data members could include the coordinates of the center of the ellipse, the semimajor axis (half the long diameter), the semiminor axis (half the short diameter), and an orientation angle giving the angle from the horizontal coordinate axis to the semimajor axis. Also, the class could include methods to move the ellipse, to return the area of the ellipse, to rotate the ellipse, and to scale the semimajor and semiminor axes: class Ellipse { private: double x; // x-coordinate of the ellipse's center double y; // y-coordinate of the ellipse's center double a; // semimajor axis double b; // semiminor axis double angle; // orientation angle in degrees public: void Move(int nx, ny) { x = nx; y = ny; } virtual double Area() const { return 3.14159 * a * b; } virtual void Rotate(double nang) { angle = nang; } virtual void Scale(double sa, double sb) { a *= sa; b *= sb; } }; Now suppose you derive a Circle class: class Circle : public Ellipse { }; This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. Although a circle is an ellipse, this derivation is awkward. For example, a circle only needs a single value, its radius, to describe its size and shape instead of having a semimajor axis (a) and semiminor axis (b). The Circle constructors can take care of that by assigning the same value to the a and b members, but then you have redundant representation of the same information. The angle parameter and the Rotate() method don't really make sense for a circle, and the Scale() method, as it stands, can change a circle to a noncircle by scaling the two axes differently. You can try fixing things with tricks, such as putting a redefined Rotate() method in the private section of the Circle class so that Rotate() can't be used publicly with a circle, but, on the whole, it seems simpler to define a Circle class without using inheritance: class Circle // no inheritance { private: double x; // x-coordinate of the circle's center double y; // y-coordinate of the circle's center double r; // radius public: void Move(int nx, ny) { x = nx; y = ny; } double Area() const { return 3.14159 * r * r; } void Scale(double sr) { r *= sr; } }; Now the class has only the members it needs. Yet this solution also seems weak. The Circle and Ellipse classes have a lot in common, but defining them separately ignores that fact. There is another solution, and that is to abstract from the Ellipse and Circle classes what they have in common and place those features in an abstract base class. Next, derive both the Circle and Ellipse classes from the ABC. Then, for example, you can use an array of base-class pointers to manage a mixture of Ellipse and Circle objects (that is, you can use a polymorphic approach). In this case, what the two classes have in common are the coordinates of the center of the shape, a Move() method, which is the same for both, and an Area() method, which works differently for the two classes. This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. Indeed, the Area() method can't even be implemented for the ABC because it doesn't have the necessary data members. C++ has a way to provide an unimplemented function by using a pure virtual function. A pure virtual function has = 0 at the end of its declaration, as shown for the Area() method: class BaseEllipse // abstract base class { private: double x; // x-coordinate of center double y; // y-coordinate of center public: BaseEllipse(double x0 = 0, double y0 = 0) : x(x0),y(y0) {} virtual ~BaseEllipse() {} void Move(int nx, ny) { x = nx; y = ny; } virtual double Area() const = 0; // a pure virtual function } When a class declaration contains a pure virtual function, you can't create an object of that class. The idea is that classes with pure virtual functions exist solely to serve as base classes. For a class to be a genuine abstract base class, it has to have at least one pure virtual function. It is the = 0 in the prototype that makes a virtual function a pure virtual function. In this case the function had no definition, but C++ does allow even a pure virtual function to have a definition. Now you can derive the Ellipse class and Circle class from the BaseEllipse class, adding the members needed to complete each class. One point to note is that the Circle class always represents circles, while the Ellipse class represents ellipses that also can be circles. However, an Ellipse class circle can be rescaled to a noncircle, while a Circle class circle must remain a circle. A program using these classes will be able to create Ellipse objects and Circle objects, but no BaseEllipse objects. Because Circle and Ellipse objects have the same base class, a collection of such objects can be managed with an array of BaseEllipse pointers. Classes like Circle and Ellipse sometimes are termed concrete classes to indicate that you can create objects of those types. This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. In short, an ABC describes an interface using a least one pure virtual function, and classes derived from an ABC use regular virtual functions to implement the interface in terms of the properties of the particular derived class. Applying the ABC Concept You'd probably like to see a complete example of an ABC, so let's apply the concept to representing the Brass and BrassPlus accounts, starting with an abstract base class called AcctABC. This class should contain all methods and data members that are common to both the Brass and the BrassPlus classes. Those methods that are to work differently for the BrassPlus class than they do for the Brass class should be declared as virtual functions. At least one virtual function should be a pure virtual function in order to make the AcctABC class abstract. Listing 13.11 is a header file declaring the AcctABC class (an abstract base class), the Brass class, and the BrassPlus class (both concrete classes). To facilitate derived class access to base class data, the AcctABC provides some protected methods. These, recall, are methods that derived class methods can call but which are not part of the public interface for derived class objects. It also provides a protected member function to handle the formatting previously performed in several methods. Also, the AcctABC class has two pure virtual functions, so it is, indeed, an abstract class. Listing 13.11 acctabc.h // acctabc.h bank account classes #ifndef ACCTABC_H_ #define ACCTABC_H_ // Abstract Base Class class AcctABC { private: enum { MAX = 35}; char fullName[MAX]; long acctNum; This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. double balance; protected: const char * FullName() const {return fullName;} long AcctNum() const {return acctNum;} ios_base::fmtflags SetFormat() const; public: AcctABC(const char *s = "Nullbody", long an = -1, double bal = 0.0); void Deposit(double amt) ; virtual void Withdraw(double amt) = 0; // pure virtual function double Balance() const {return balance;}; virtual void ViewAcct() const = 0; // pure virtual function virtual ~AcctABC() {} }; // Brass Account Class class Brass :public AcctABC { public: Brass(const char *s = "Nullbody", long an = -1, double bal = 0.0) : AcctABC(s, an, bal) { } virtual void Withdraw(double amt); virtual void ViewAcct() const; virtual ~Brass() { } }; //Brass Plus Account Class class BrassPlus : public AcctABC { private: double maxLoan; double rate; double owesBank; public: BrassPlus(const char *s = "Nullbody", long an = -1, double bal = 0.0, double ml = 500, double r = 0.10); This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. BrassPlus(const Brass & ba, double ml = 500, double r = 0.1); virtual void ViewAcct()const; virtual void Withdraw(double amt); void ResetMax(double m) { maxLoan = m; } void ResetRate(double r) { rate = r; }; void ResetOwes() { owesBank = 0; } }; #endif The next step is to implement those methods that don't already have inline definitions. Listing 13.12 does that. Listing 13.12 acctABC.cpp // acctabc.cpp bank account class methods #include <iostream> #include <cstring> using namespace std; #include "acctabc.h" // Abstract Base Class AcctABC::AcctABC(const char *s, long an, double bal) { strncpy(fullName, s, MAX - 1); fullName[MAX - 1] = '\0'; acctNum = an; balance = bal; } void AcctABC::Deposit(double amt) { if (amt < 0) cout << "Negative deposit not allowed; " << "deposit is cancelled.\ n"; This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. else balance += amt; } void AcctABC::Withdraw(double amt) { balance -= amt; } // protected method ios_base::fmtflags AcctABC::SetFormat() const { // set up ###.## format ios_base::fmtflags initialState = cout.setf(ios_base::fixed, ios_base::floatfield); cout.setf(ios_base::showpoint); cout.precision(2); return initialState; } // Brass methods void Brass::Withdraw(double amt) { if (amt < 0) cout << "Negative deposit not allowed; " << "withdrawal canceled.\ n"; else if (amt <= Balance()) AcctABC::Withdraw(amt); else cout << "Withdrawal amount of $" << amt << " exceeds your balance.\ n" << "Withdrawal canceled.\ n"; } void Brass::ViewAcct() const { ios_base::fmtflags initialState = SetFormat(); This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. cout << "Brass Client: " << FullName() << endl; cout << "Account Number: " << AcctNum() << endl; cout << "Balance: $" << Balance() << endl; cout.setf(initialState); } // BrassPlus Methods BrassPlus::BrassPlus(const char *s, long an, double bal, double ml, double r) : AcctABC(s, an, bal) { maxLoan = ml; owesBank = 0.0; rate = r; } BrassPlus::BrassPlus(const Brass & ba, double ml, double r) : AcctABC(ba) // uses implicit copy constructor { maxLoan = ml; owesBank = 0.0; rate = r; } void BrassPlus::ViewAcct() const { ios_base::fmtflags initialState = SetFormat(); cout << "BrassPlus Client: " << FullName() << endl; cout << "Account Number: " << AcctNum() << endl; cout << "Balance: $" << Balance() << endl; cout << "Maximum loan: $" << maxLoan << endl; cout << "Owed to bank: $" << owesBank << endl; cout << "Loan Rate: " << 100 * rate << "%\ n"; cout.setf(initialState); } void BrassPlus::Withdraw(double amt) { This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it. Thanks. [...]... provide read-only access to the fullName and acctNum data members and make it possible to customize ViewAcct() a little more individually for each derived class This new implementation of the Brass and BrassPlus accounts can be used in the same manner as the old one, for the class methods have the same names and interfaces as before For example, to convert Listing 13.10 to use the new implementation, you . cout.setf(initialState); } // BrassPlus Methods BrassPlus::BrassPlus(const char *s, long an, double bal, double ml, double r) : AcctABC(s, an, bal) { maxLoan = ml; owesBank = 0.0; rate = r; } BrassPlus::BrassPlus(const. virtual ~Brass() { } }; //Brass Plus Account Class class BrassPlus : public AcctABC { private: double maxLoan; double rate; double owesBank; public: BrassPlus(const char *s = "Nullbody",. }; Then the BrassPlus class could access balance directly without using Brass methods. For example, the core of BrassPlus::Withdraw() could be written this way: void BrassPlus::Withdraw(double